Gas engine ignition system



D. a. 50m:

GAS ENGINE IGNITION SYSTEM Filed Dec.

min

' INVENTOR Dan/A 1:0 IT'Ba HIV Patented Dec. 13, 1949 UNITED STATES PATENT OFFICE GAS ENGINE IGNITION SYSTEM Donald I. Bohn, Pittsburgh, Pa.

Application December 21, 1945, Serial No. 636,453

2 Claims.

This invention relates to ignition systems for internal combustion engines and the like.

The object of th invention is to provide an ignition system in which the firing discharge of the spark .plugs will be regulated and restricted to give eflicient sparking while avoiding excessive erosion of the spark plug terminals.

In the accompanying drawings illustrating the invention Fig. 1 is a diagram of a conventional ignition circuit, and

Fig. 2 is a cross sectional view of a detail.

Upon firing of a spark plug there may be excessive rush of current capacitativelystored up in the connected parts of the system, and the intensity of the resulting discharge wastes the terminal points and greatly reduces the life of the plug to only a fraction of its normal endurance.

Various proposals to overcome this have been objectionable either as involving expensive and cumbersome accessories or as working only a partial cure still subjecting the terminals to excessive discharge over and above that required for firing the gaseous mixture.

I have found that if theampere discharge rate is limited to predetermined small value, the discharge is effective in igniting the gases and at the same time so limited in intensity of current as to avoid excessive erosion of the terminals and correspondingly extend the life of the spark plug. To attain this there is interposed at the spark plug a high resistance in series with the terminal gap. This high resistance operates to bring the full static E. M. F. to the terminals and then upon sparking acts to restrict the rate of current discharge to only a very minor part of the rate of now which would otherwise occur. For instance, assuming a high tension system which reaches 15,000 volts when the plug fires and assume a substantially infinite amount of stored energy in the distribution system, a resistor of 500,000 ohms will prevent the current from exceeding .030 ampere which is but a small fraction of the current. which would normally flow with appreciable lead capacity and no resistance, as constituted in previous practice. With these conditions stated, the resulting low amperage discharge across the spark plug gap is sharp and fully eflective to ignite the explosive mixture.

The control resistance in ohms is preferably more than ten times the voltage reached at the establishing of the spark so that the current permitted is less than one tenth of an ampere and the resistance may be even higher giving currents of only a few hundredths of an ampere. In general, the range of resistances may be 300,000 to 1,500,000 ohms with 500,000 to 1,000,000 ohms generally preferred.

A conventional ignition system, aside from the special resistor which is the basis of this present control, is shown in Fig. 1'. With this system, a battery I supplies primary winding 6 of spark coil 2 at timed intervals through manual switch 3 and breaker points 4, which are shunted by condenser l to prevent sparking and which is suitably driven from cam 5. Secondary 8 of spark coil 2 is grounded at 9 and connected at the other side to the arm ill of distributor H having a contact for each plug, one of such contacts I! being shown connected by lead or cable It through resistor It to the central electrode ii of the spark plug IS, the shell of which is grounded at H for return to the grounded side of the secondary 8. The cable I 3 is of conventional type embodying a low resistance metallic conductor.

As is well known in the art, when breaker points 4 part, the sudden collapse of flux in the core of spark coil 2 induces a high voltage in the secondary winding 8 which has many times the turns of primary winding 6, producing a peak voltage of sufficient magnitude to jump across a spark plug gap under operating conditions. In multiple cylinder engines, the connection from secondary coil 8 to spark plug I8 is usually effected by means of the conventional high tension distributor II which selects the proper spark plug to be fired at the proper time.

It is physically impossible to arrange a conducting circuit from spark coil secondary 8 to spark plug l8 without such a circuit involving capacitance to ground which is in efiect capacitance shunted around the spark plug terminals. Under such conditions, and leaving out for a moment resistor H, the stored energy in such lead capacitance causes a relatively high rate of cur-- rent discharge across the points of spark plug I 6, when the plug I8 is caused to fire.

As previously mentioned, such a high rate of current flow across the spark plug points causesundesirably rapid erosion of the spark plug points. This peak current may be many times greater than that obtained by grounding or shorting out the high tension lead or cable to the plug. In fact, the peak current may represent many times the short circuit amount by reason of the capacity existing between the cable and the engine which stores energy and dis- 3 charges exceedingly rapidly through the spark plug and imposes on the plug for an instant a higher current value than a magneto or battery actuated ignition coil could supply.

To overcome this undesirable rapid erosion of the spark plug points, I provide a resistance It located physically directly adjacent to spark plug it and in series with its terminal.

since all of the harmful stored energy in the high tension lead system must flow through this resistor before jumping across the points of spark plug it, it is obvious that the rate of current flow cannot exceed a value represented by Ohms law.

In other words, assuming that spark ccil sec-' ondary 8 can maintain 15,000 secondary volts during the period of firing, and further assuming that resistor It has a value of 500,000 ohms, it is obvious that under no circumstances can the current through the spark plug exceed .030 ampere, and such maintenance of voltage of secondary 8 cannot strictly exist, the current will actually be something less than this figure which represents a theoretical maximum. It is obvious that resistor it does not appreciably change the total number of ampere-seconds which will be caused to jump across the spark plug points, but tests have conclusively indicated that when this given number of ampere-seconds is disposed of in an extremely short period of time, and therefore at a relatively high rate, the spark plug erosion is much greater than if the same number of ampere-seconds are permitted to fiow across the spark plug gap at a lower rate and therefore for a longer period of time. This controlled rate of discharge of the accumulated energy has been found critically to increase the life of the spark plug terminals by many times. In some cases the normal erosion under high discharge is so rapid as to limit the spark plug life to 200 to 300 hours in service. whereas with the lowering of the rate of discharge by the resistors of the present system, the useful service of the spark plug terminals is extended to 2,000 to 3,000hours.

with the very high resistance It interposed between the end of the cable and the spark plug, the current surge is restricted to a safe amount while at the same time an effective igniting spark is provided between the spark plug electrodes, this type of discharge lending itself to the control of the flow to provide efiicient firing without substantially exceeding the amount eilective in the ignition. l v

The resistor It may be interposed between the cable and the spark plug in any convenient manner, for instance as illustrated in Fig. 2 where the resistance is contained in the cylinder 22 rigidly" clamped in the terminal heads Ii, 23 to form a unit attached at one end to the spark plug l8 and at the other to the terminal 24 of the cable It.

While the invention has been illustrated in connection with a showing of a particular circuit, it is not limited thereto, and various modifications may be adopted.

The resistance It may be made variable within a predetermined range so as to be adjustable to 4 the special characteristics or individual installations depending on the generating current and the capacity efleots in the high tension circuit.

Other modifications may be resorted to maintaining the advantages of the system shown and described, and the principle of the invention is intended to cover such combinations as fall within the scope of the appended claims.

I claim:

1. An ignition system for a spark fired gas engine comprising a source of high potential ignition impulses, a spark plug having electrodes providing a spark gap, a high tension distribution circuit including a high tenson cable connecting said source and said spark plug electrodes and having appreciable electrostatic capacity producing a substantially instantaneous peak current discharge across said spark gap at the instant of firing in excess of the ignition current normally supplied by said source for firing, and a current-limiting resistor of a fixed value of between 300,000 and 1,500,000 ohms connected in series with said cable adjacent to said electrodes and acting continuously to prevent current flow across said gap from exceeding a value represented by Ohms law, whereby discharge of capacitatively stored energy across said gap is profiring in excess of the ignition current normally supplied by said source for firing, and a currentlimiting resistor of a fixed value of between 500,000 and 1,000,000 ohms connected in series with said cable adjacent to said electrodes and acting continuously to prevent current fiow across said gap from exceeding a value represented by Ohms law, whereby discharge of capacitatively stored energy across said gap is prolonged and rendered harmless to said electrodes. 1 DONALD I. BOHN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS 

